1. Field of the Invention
In its most general aspect, the invention relates to a system for carrying out a process by sensing the conductivity of a fluid, the process being carried out in different modes dependent on the conductivity of the fluid, and for regulating and stabilizing the power supply to provide a current level for each mode which current level is appropriate to prevailing conditions.
In a specific case, the invention relates to a silver recovery system being operable in either a standby mode, having associated with it a predetermined standby current level, or a plating mode, having associated with it a predetermined plating current level. More specifically, the invention relates to such a silver recovery system wherein there is provided a standby voltage level, for causing the standby current level, and a plating voltage level, for causing the plating current level, and means for regulating the standby voltage level and the plating voltage level respectively in accordance with prevailing conditions.
In the present disclosure, the system is described as being embodied in a silver recovery system.
2. Description of Prior Art
Systems for recovering metals from solution by an electrolytic process as well known in the art, and an interesting and informative history and description of such systems is given at columns 1 and 2 of U.S. Pat. No. 4,776,931, Hardy, Oct. 11, 1988. In this patent, Hardy also describes his own system wherein a plating voltage is reduced to a lower standby voltage if the plating current falls below a threshold value. The standby voltage is periodically increased to its higher plating value for brief intervals to test the current at the higher voltage during these intervals. If the current drawn during any interval is higher than the threshold current level, then the voltage remains at the plating voltage. If it is below the threshold, then the voltage once again drops to the standby level until another sample is to be taken.
The problem with the Hardy system is that both the standby voltage and the plating voltage are predetermined and remain unvaried in spite of prevailing conditions.
This two-stage high-voltage/low-voltage system assumes that conditions are such that the high voltage will always deliver a predetermined plating current, and that the low voltage will deliver a predetermined standby current. However, this does not take into account deterioration of the plating cell occasioned by, for example, electrode oxidation, pH variants, sulphite levels, flow rates, etc. due to which a predetermined voltage will not necessarily, after time, deliver the same predetermined current that it did before the onset of deterioration. These points are also discussed to some extent in the Hardy patent.
U.S. Pat. No. 4,612,102, Brimo et al, Sept. 16, 1986, also teaches a two-stage high-voltage/low-voltage silver recovery system. In the Brimo et al patent, the conductivity of the electrolyte is monitored (by monitoring the current flow) and the driving voltage is set to either a plating voltage or a standby voltage depending on the state of the conductivity of the current. Brimo et al, as Hardy, also assumes that a predetermined voltage will always cause a predetermined current to flow which, as above pointed out, is untrue. The Brimo et al patent is further discussed in columns 1 and 2 of the Hardy patent above referred to.
Other silver recovery systems known in the art are described in, for example, U.S. Pat. No. 4,762,598, Drew, Aug. 9, 1988, U.S. Pat. No. 4,675,085, Vasquez, Jun. 23, 1987 U.S. Pat. No. 4,619,749, Nusbaum, Oct. 28, 1986, and U.S. Pat. No. 5,102,513, Pelkus, Apr. 7, 1992.
The '598 patent describes a silver recovery system which provides a means for counter-acting the ripple of the plating current by providing a current which is sufficient to maintain plating but not high enough to permit the formation of silver sulphide. This is accomplished by controlling the mean value of the current.
In the '085 silver recovery system, the anode and cathode are maintained in a fixed spaced relationship in a casing. The plating voltage, which will be increasing due to increase in resistance of the is monitored until it reaches a predetermined reference value. At that time, a drain is opened to drain the metal containing solution from the casing.
The '749 patent teaches a silver recovering system which has both primary and secondary electrodes. The plating current is correctively changed by detection logic in response to excessive variation in the electrolytic resistance of the liquid.
Pelkus teaches, in the '513 patent, the step of monitoring current and voltage to determine whether there is sufficient silver in solution to continue silver recovery. When the silver of below a predetermined value, a "lock-out" condition is triggered. During this period, no voltage is applied except when sampling. In this regard after "lock-out", further silver will be added to the solution and the content of silver in solution will be monitored at sampling intervals.
As can be seen, none of the references, or any other references known to Applicant, or any systems known to Applicant, provide means for regulating the voltage in the different modes of operation to provide predetermined and desired current levels at these different modes.